The phase in a woman’s reproductive lifecycle leading up to menopause. Menopause is reached when a woman hasn’t had a period for 12 months in a row. Before that point, during perimenopause, a woman's body slowly makes less of the hormones estrogen and progesterone. This causes some women to have symptoms such as hot flashes and changes in their periods. Many women go through it in their 40s and 50s.